the height of the tool point should be exactly level with the
centre line of the work; for the section A B of a cone, Fig.
788, through the axis, is a triangle, for which a straight-line
feed would be suitable, but the section at CD, below the axis,
is a hyperbola, and a straight-line feed would simply produce
a reversed hyperbola instead of a true cone.
P. 249. Tooling Circular Arcs.—The methods of tooling
arcs of large radius may thus be classified :—
1. Milling or slotting, with hand feed.
2. Milling in a profiling machine having a curved copy.
3. Special milling apparatus, p. 752, using a property of the
4. Planing on a pivoted table controlled by a rod equal in
length to arc radius.
5. Turning on a large face plate in a vertical lathe, whose
short mandrel is sunk in the ground.
Numbers i, 2, and 3 have already been described. No. 5 is
practised at Woolwich on racers or roller paths of large radius.
But as large arcs are not often required, it is better to fit up a
planing machine in the manner shewn at Fig. 789, where two
positions are given. Taking position L, let a line A B be drawn
immediately under the tool point, and let a stud B be fixed to the
table. Next, let a triangular frame A CD, one side of which is
the small table c D, be pivoted on stud B, and further pinned
at A; the hole in CD being slotted to permit the planing table to
travel, The work to be planed is now fastened to c D, and the
planing commences; then, by referring to position II., it will be
found that the curve c D, struck from A, will always lie under the
tool point whatever the table position.